1. Field of the Invention
The present invention relates to a circuit device for driving an alternating current (AC) load.
The invention relates, particularly but not exclusively, to a circuit device which is inserted between a terminal of an AC power supply line and a terminal of an electric load to be driven, and includes a generator of PWM signals to be transferred to the load.
2. Description of the Related Art
As it is well known in the relevant art, the simplest way of varying the supply voltage of a single-phase AC electric motor, or to a resistive/inductive load, is that of using a partialized phase Triac device effective to partialize the supply voltage to the motor.
Briefly, for varying the voltage applied to an AC electric load, it has been known to drive electric motors with the sinusoid of the supply voltage partialized by controlling the conduction angle of a thyristor.
This is a cost-effective technique that has been in use for decades, but has a problem in that harmonics are introduced in the current waveform of the power supply line, resulting in a low efficiency of the whole system.
European standards, e.g., Standard No. EN 61000-3-2, place strict limitations on the harmonic contents of line current, and the use of circuit devices that are based on bridge structures producing PWM (Pulse Width Modulation) signals, known for being less “fouling” in terms of introducing harmonics, provides at present a much-favored solution.
The expanded use of non-linear loads, with its attendant increase of harmonic currents in the supply line, in recent years has led regulatory technical boards to set upper limits on the harmonic contributions from industrial and household appliances.
Accordingly, the manufacturers of such appliances are to adjust the specifications for their ranges to bring them within the new standard of operation.
It has been proposed to solve the problem by using more or less complicated and costly inverter systems, adapted to lower the harmonic contents of currents with respect to the load. Circuits of this kind employ inverter-type DC/AC switches that allow a sinusoidal voltage to be output, the amplitude and frequency of which can be varied individually. These prior art circuits are based on bridge circuits adapted to generate PWM signals. The DC voltage is conventionally obtained by rectifying and filtering the supply line voltage.
For instance, a rectifying diode is connected in parallel to a filter capacitor and a switch having its ends connected to the electric motor or to any electric load, such as a lamp or an electric oven. An approach of this type is described in the European Patent Application No. 98830809.4, for example.
However, not even these approaches have succeeded to lower the harmonic contents with respect to the supply line, because of the presence at the input of supply stages that employ rectifying bridges and filter capacitors, both introducing current distortions.
In all cases, inverter systems involve double conversion, AC to DC at the input and DC to AC at the output, as well as the use of complex modulation and control techniques normally implemented by microcontrollers that raise the system cost unacceptably in low-end applications.
The underlying technical problem of this invention is to provide a circuit device for driving AC electric loads, which device should have appropriate structural and functional features to allow a direct connection of the load to the AC power supply line, and to drastically reduce the likelihood of current harmonics being introduced in the power supply line, thereby overcoming the limitations and obviate the drawbacks of the solutions provided by the prior art.